‘Jesus heard that they had excluded him, and finding him he said, “Do you believe in the Son of Man?”
Jesus, having heard that the man had been excluded from the synagogue, now sought him out. They had thought that they were excluding him from the worship of God, not realising that they were rather throwing Him into the hands of God. Jesus then asked him if he ‘believed in the Son of Man'. Ideas about the heavenly Son of Man were current at the time (see John 3:13; John 5:27; John 6:62 which mirror popular belief), a figure who came from God and would one day receive from God glory, dominion and power and participate in the judgment. So Jesus expected him to understand.
Many good authorities have here ‘the Son of God'. If so Jesus may have been probing to find out his views about heavenly figures in mind in Jewish tradition. But ‘Son of Man' probably has the weight on its side.
Either way the terminology refers to One Who was to come from God and would be empowered to act on His behalf.